1. Field of the Invention
This invention relates to a cement which can be used for repair of osseous structures, fixation of implants, and in particular as a dental or orthopedic cement.
2. Description of the Prior Art
Considerable research effort has been directed to providing cements which have adhesion for, and can be used for repairing human and animal bones or teeth. Although a wide variety of different cement compositions has been proposed for this purpose, the most successful are the acrylic cements, particularly those cements derived from polymers of methyl or ethyl acrylate, or methacrylate.
In the usual method, a metal implant or pin is inserted into the bone which is held in place by the use of a cement which bonds the implant to the adjacent bone. The acrylic monomer cements generally consist of a relatively thick paste or organosol of polymer powder and a low temperature polymerization initiator dispersed into the liquid monomer.
Several severe difficulties, however, have now been encountered which threaten to significantly hinder the full development of this technique. For one, the polymerization of the acrylic monomer is accompanied by the evolution of a considerable amount of exothermal heat which reportedly has been responsible for the development of bone temperatures as high as 83.degree. C. using conventional orthopedic cements. [See for instance, Homsy, C. A., Prosthesis Seating Compounds of Rapid Cure Acrylic Polymers, National Academy of Sciences -- American Academy, Orthopedic Surgery, Joint Workshop on Total Hip Replacement and Skeletal Attachment (Nov. 6, 1969), and Bloch et al, Evaluation of Cold-Curing Acrylic Cement for Prosthesis Stabilization, Clin. Orthopaedics 72, 239 (1970).]
This high heat evolution has deleteriously resulted in necrosis of the surrounding bone tissue [See Wiltse et al, Experiment Studies Regarding the Possible Use of Self-Curing Acrylics in Orthopedic Surgery, J. Bone Joint Surg. 35A, 961 (1957).]
Another difficulty encountered has been the observation that a small amount of unreacted acrylic monomer invariably leaves the bonding area and is released into the general circulatory system. Since the acrylic monomers are quite toxic, even in relatively low concentration, this is now recognized as a considerable difficulty which threatens the continued usage of this technique.
A still further difficulty encountered, is that the long-term adhesion of the cement is disappointingly low, so that frequently the implant has been loosened, which resulted in the necessity of further surgical procedures. Methods therefore have been sought to improve the long-term adhesiveness of the cement.
The implant technique for orthopedic and certain dental procedures was first developed by Charnley, A Biomedical Analysis of the Use Of Cement to Anchor the Femoral Head Prosthesis, J. Bone Joint Surg. 47B 354 (1965). At that time, Charnley recognized that the success of the technique depended upon the smooth transition from implant to bone through a thin layer of fibrous tissue. If the thin fibrous tissue layer fails to develop, loose prosthesis can occur, which can result in bone erosion or migration.
To overcome this difficulty, it has been attempted to provide means to promote the growth of the fibrous tissue. Several orthopedic surgeons have attempted to fabricate prosthetic bone implants having a configuration of natural bone, i.e., composed of a network into which the tissue can grow. Incorporating pores in ceramic material has been successful in that the growth of fibrous tissue and natural bone into the implant can occur. By incorporating pores in ceramic materials, the ingrowth of fibrous tissue followed by ossification has been observed to occur, provided the pores are continuous and large enough to accommodate blood vessels. The resulting bone ingrowth can then rearrange along the stress lines. [See Hulbert et al, Potential of Ceramic Materials as Permanently Implantable Skeletal Prosthesis, Second Materials Engineering Conference, National Meeting of AlCh.E (1970)].
However, all of the currently available implants are pre-formed and thus do not fit exactly into the bones in which they are implanted, necessitating the use of a cement to hold them in place. The previously available cements, however, tended to destroy the available porosity into which tissue could grow.
Although certain of these difficulties have been partially overcome in the prior art, no prior art technique suggested has been found to overcome all of these difficulties.
For instance, it is the recognition of this invention, as will be explained below, that the incorporation of an adequate amount of particulate water soluble materials will provide a cement with a sufficient porosity through which the fibrous tissue can grow to strengthen the implant-to-tissue bond. The mere incorporation of a soluble chemical into acrylate polymers such as polymethylmethacrylate to provide porosity has been disclosed in U.S. Pat. No. 2,347,567. In that reference, however, the monomer and a germicide in crystalline form are charged into a mold in which polymerization is effected to form a surgical implant. The implant is formed in the shape of a screw which is screwed into the bone. In turn, the body fluid dissolves the crystalline germicide so that a porous structure is obtained into which osseous bridges can be formed. This is unlike the present invention which is concerned with the formation of a cement which is used to fix the implant, and not to the implant per se. In the present invention the particles are incorporated into the monomer or partially polymerized resin to form a cement which is polymerized in the body -- not in a mold. The particles then function not only to provide a porous bond interface, but, as will be explained below, also to prevent thermal necrosis of surrounding bone tissue, and to prevent out-migration of the monomer. Moreover, the product of this patent was germicidal crystals which would burn the surrounding tissues, and hence would be detrimental for implant purposes. That is, germicidal crystals would be toxic to the surrounding tissue, and hence unsuitable. Moreover, some of the germicide will invariably remain in the mixture after the germicide has become leached out, which would prevent growth of tissue into the pores of the cement. One object of the present invention, as described in detail below, is to provide for tissue growth into the osseous cement, and thereby improve the longevity of the bond.
U.S. Pat. No. 3,215,137 discloses the use of sucrose esters as a liquid diluent to prevent excessive heat generation during polymerization of methylmethacrylate, which is used for immobilizing bandages for fractured limbs; however, soluble particles are not used, and the use contemplated is outside the body and not internal.
None of these references suggest a method of simultaneously providing a cement which permits the ingrowth of tissue so as to provide a high strength porous bond, elimination of monomer toxicity, and elimination of thermal necrosis.
Accordingly, a need exists for a cement which can be used for orthopedic and dental applications, which does not release toxic quantities of monomer into the general circulatory system, which does not cause thermal necrosis during polymerization and hardening, and which provides a porous high strength interface as a result of tissue ingrowth.